Serving Section 8 for rent arrears - time query

Possession - Section 8 and Section 21

Could someone possibly clarify when a section 8 for rent arrears can be served?

Having read numerous things they all say "when the tenant is 2 months in arrears". Is this once 2 payments have been missed or once 2 full months have passed effectively being 3 months behind?

To explain for an example;

If the rent was due on 1st November and missed, the next payment was due on the 1st December and again missed can a section 8 be served on/after 2nd December or would it be 31st December?

Apologies for the probably very obvious question but quiet new to all this and living a nightmare currently.